Macrophages are essential to innate immunity to infections. Activation of macrophages by lipopolysaccharide (LPS) and cytokines such as interferon-? (IFN?) and tumor necrosis factor-? (TNF?) increases their microbicidal activities but also increases damage to tissues due to inflammation. As therapies which target chronic inflammation leave patients vulnerable to infections, new strategies are needed that can selectively increase macrophage antimicrobial activities. The long-term goal of this research is to devise such strategies through investigations of fundamental macrophage cell biology. This lab discovered recently that exposure of murine macrophages to bacteria, LPS, IFN? or TNF? leads to stabilization of their lysosomes against mechanical damage, a phenomenon termed inducible renitence or IR. As vacuolar membrane damage is essential to the virulence of many pathogenic microbes, to infection by viruses and to inflammation by micro-articulates, this novel phenomenon could potentially be exploited therapeutically. The objective of the present work is to define the cellular and molecular basis of inducible renitence. The central hypothesis is that renitence is induced by classical activation and consists of enhanced mechanisms of membrane damage-repair. The experimental model for these studies is a system in which macrophage lysosomes or phagolysosomes are subjected to controlled levels of physical perturbation, which allows quantitative evaluation of mechanisms that resist or repair damage. The central hypothesis will be tested by addressing three specific aims. The first aim will determine the conditions and factors which induce renitence in human and murine macrophages. Renitence will be measured in classically activated macrophages, alternatively activated (wound-healing) macrophages and regulatory macrophages, as well as macrophages treated with other agents. The second aim will determine the role of membrane damage-repair mechanisms in renitence. The kinetics of phagolysosome damage and repair will be measured and the contributions of vacuolar calcium, synaptotagmin VII and acid sphingomyelinase to renitence will be analyzed. The third aim will determine the role of renitence in macrophage resistance to infection by the Gram-positive intracellular pathogen Listeria monocytogenes, which normally scapes into cytoplasm by damaging vacuolar membranes. By defining the cellular and molecular basis of IR, this research will introduce a new strategy for manipulation of macrophage function. Therapies which increase renitence selectively could reduce inflammation due to micro-particulates or benefit immunosuppressed patients and individuals with chronic inflammatory diseases, such as therosclerosis and Crohn's disease.